Abandoning its useless attacks against Rohuzar, the creature rotates in the passage and turns its tentacles against Raqisat.  

Its first two strikes miss terribly... but its final tentacle strike coils around him, barbs protruding into his flesh and doing 16 piercing damage (before any rage reduction) and he is grappled (escape DC 14). Until this grapple ends, the target is restrained, and the monster can’t use the same tentacle on another target. The monster has ten nine tentacles.

Round Three: 

1. Raqisat
2. Aubrey 
3. DK
4. Cosette 
5. Grugga and friends 
6. Leelan
7. Rohuzar (protection)

Con save (maintaining branding smite on the monster): 13 (success) (rolled in game log)

Seeing Raqisat in a bad spot after being splashed by lightning blood and coiled in organic barbed wire, Grugga pulls out his tiny musicbox-looking contraption and grinds a purple crystal into mist that wraps around Raqisat, scabbing over some of the wounds with purple crystal flanks. cure wounds: 12 healing (rolled in game log). (Action).

Not seeing a safe way in, Grugga retreats back around the corner where he was before charging in. As he presses himself against the wall, he whistles for Zap to move into the hallway behind Raqisat. (Bonus action). Zap will use its reaction to give the monster disadvantage against its next attack against Raqisat (steel defender ability).

Rohuzar sees Raqisat getting it bad and tries to leap up and chop at the tentacle holding him

Chop-chop!

Attack: 10 Damage: 11 

Attack: 13 Damage: 12 

Leelan grimaces at the unfortunate friendly fire and calls out.

"Apologies, friend! It won’t happen again."

He then fires another volley of arrows.

Attack 1: Attack: 25 Damage: 3

Attack 2: Attack: 18 Damage: 4

Slayer’s Prey on the first attack that hits the creature: 5

“Just between you an’ eye, he could use some adult supervision!” Cos calls out down the hall.

Action: Vicious Mockery Wis ST DC15
     On fail: 7 psychic damage and have disadvantage on the next attack roll it makes before the end of its next turn.

Cosette begins to hum in a low ominous baritone, underlining the sounds of battle. She builds the haunting discordant melody with stomps of her foot, thumping and scrapes against the wall to further terrorize the aberrations psyche.

(Essentially ‘prepping’ a reaction ahead of time so it works smoother for pbp:)

Save 17

re save due to silvery barbs: 10 less bane: 2

[ahem]

One of the tentacles raises and a disgusting, lamprey like mouth full of corkscrew teeth opens at the terminal end.  The horrific little mouth speaks to Cosette in a gurgling, deeply upsetting voice, "Sticks and stones may break my bones but names will never hurt me, bloodbag."

[That brings us around to the top of Round 4.  Raqisat and Aubrey are up.  Raq can either spend an action to try and break free, ending his turn, or he can attack or take some other action if he chooses.  He already has advantage due to rage on strength checks (if he wants to use Athletics to escape the grapple/restraint) but he now also has advantage on his next attack or other check.  So he could opt to...

• Try to escape using DEX (Acrobatics) and apply advantage.
• Attack, which is at disadvantage due to restraint, but he could apply the advantage from Silvery barbs to cancel that out on his first attack roll and roll it without advantage or disadvantage.  An off hand follow up attack would still be at disadvantage.
• Something else, I guess but can't think of anything else.

Just putting this out there pre-emptively so you know I would rule on the application of Silvery Barbs here.]

(Rage rd 4, Bard Insp avail & advantage on next attack…)

     “It’s no worse than a hug from that weird uncle,” Raqisat says to assure… himself?  The others?  It’s unclear, really.  He does, however, allow himself to continue to be grappled for the time being feeling it’s an opportune time to strike out!

Whip attack:  Natural 1….  For 7 damage.  Maybe whipping while grappled is bad?

Bonus Action whip attack:  10 to hit, after Dis… Will add Bardic Inspiration to maybe hit?  Now:  16 to hit, 4 damage plus 3 sneak attack damage…

Rohuzar continues his attacks on the creature, but it has transitioned to full-defense against him, feinting and swatting at his axe with its spare tentacles and evading anything more than scratches.  

Leelan and Aubrey do better with their ranged attacks... the side of the creature facing Rohuzar is now peppered with arrows and patches of blackened, necrotic flesh.

Raqisat's first whip attack is ineffective, tangling around his own feet as the creature holds him aloft and he spins out of his own control.  Using the spin to his advantage, he gets a second whip attack off that does connect, swatting the creature painfully but dealing another 5 lightning damage back to him.

The creature looks grievously wounded now.

It spins Raqisat to face it... and then he feels the mouth at the end of the tentacle holding him open up and bite down hard.  [Raq's CON save from the discord = 4]  He feels agony throughout his body as the creature exchanges fluids with him.  He takes 24 more lightning damage.  As Raqisat's body spasms and sparks, some of the wounds on the pale beholder's body begin to heal and scab over....

[Blood Drain does not include an attack roll so there is nothing to apply disadvantage to for the Steel Defenders ability.]

Having refreshed itself on Raq, the eye turns to the next obstacle in the hall, Zap.  Its first tentacle misses, but the next catches the defender off of its guard and latches on, dealing 16 lightning damage.  Zap is also grappled (escape DC 14). Until this grapple ends, the target is restrained, and the monster can’t use the same tentacle on another target. The monster has ten nine tentacles, two of which are grappling opponents.

With the two enemies blocking it in the hall grappled by its tentacles, the creature hovers back in the direction it came [due to grappling, movement halved].  

[Rohuzar may take an attack of opportunity as it leaves his reach.]

As it reached the doorway where he is lurking, one of the tentacles rises to face Grugga.  The little mouth opens and addresses him respectfully.  "I shall depart now.  I consider these two morsels poor but adequate compensation for your rakish and uncalled for violence upon my person."

Round Four: 

1. Raqisat (grappled/restrained)
2. Aubrey 
3. DK
4. Cosette 
5. Grugga and friends (Zap grappled/restrained)
6. Leelan
7. Rohuzar (protection)

Momentarily stunned by the the fluency and eloquence of the creature from beyond the veil, Grugga nonetheless presses the attack to free Raqisat. "You'd best drop him if you want to live!" the Orc yells while slashing at the fleshy body of the aberration.

(action) (all in game log)
Attack 1: 12
Damage 1: 10 slashing
Attack 2: 23 
Damage 2: 10 slashing

"Aubrey! Leelan! Here!" He moves east 5 feet to give the others a good shot.

Grugga orders Zip back to the south wall to shoot (bonus action). Attack: 10 Damage: 6 force

Rohuzar's AoO Attack: 21 Damage: 15 

Followed by a short pursuit and more chop-chop

Attack: 10 Damage: 15 

Attack: 18 Damage: 9 

Rohuzar pursues the monster, hacking and slashing while Grugga simultaneously attacks and issues his ultimatum.

Whatever the aberration may have intended to respond with, we shall never know.  Rohuzar and Grugga land multiple strikes - in the process, Rohuzar absorbs 3 and 2 lightning damage, while Grugga takes 10 lightning damage - and it is too much.  The pale fleshly ball tears open, the single eye splashing down to the floor of the sewer as its tentacles go limp, spilling Zap and a barely alive Raqisat roughly to the ground.

The battle is over... you have slain your first aberration.

"Excellent work team." Aubrey says with a relieved smile. "Grugga, can you see to Raqisat, Leelan, perhaps bring back some samples for study, but watch out for any residual electricity, and very well done everyone, this swift resolution of our task should impress our current client." He adds with a satisfied smile, studying the slain aberration to see if it was somehow carrying anything around. "Before we head back we might consider taking a closer look at that steel door. Furvus also spotted an altar to the south dedicated to Mammon surrounded by kobold skulls that possibly might be worth taking a closer look at. Although I would understand if we prefer to simply head back to rest and recouperate now." The young noble finishes, giving Raqisat a sympathetic smile.

Perception: 17

Rohuzar will work with (Leelan I assume?) to get a solid inspection of this thing, inside and out. There are some glares at the short fellow as he occasionally limps from the pain of his friendly fire.

(Have some Help)

With the creature taken care of, Leelan rejoins the others. After making sure everyone is still standing, he’ll approach Rohuzar.

"I wish to extend my sincerest apologies once again, friend. Fighting in such enclosed space will require some getting used to I’m afraid. Here, let me help."

Cure wounds 7 points of healing.

With that out of the way, the gnome, with the help of his orc companion, will begin to investigate this creature’s anatomy and physiology. He’ll take some sample from the tentacle and perhaps some teeth as well. All of his discoveries will be compiled in his manuscript.

Investigation (with help from Rohuzar): 15

     Raqisat grunts as he is unceremoniously dumped onto the ground when released.  He rests there but a moment before slowly making his way to his feet.  “I’m good,” he insist despite being a bit unsteady on his feet.  “Wouldn’t be much use if I couldn’t give and take a few hits, now would I?”

     Raqisat uses the need to stow his whips and retrieve his blades as an excuse to regather himself and steady himself.

     “Should probably drag the whole beastie back to them,” he notes.  “Kingy may want a trophy.  Or tentacle soup…” Raqisat suggests as he steps back to the creature and takes a good long look it it.  “Ugly enough, ain’t it?”

     “Yes though,” he adds after a moment’s thought.  “We should check out the nearby areas before the rats claim it all as their own…”